The sight of seabirds following a fishing vessel is familiar to anyone who has spent time near a commercial harbor. As the crew works to butcher the fish from the back of the deck, the entrails are usually tossed overboard. And the birds are always ready for a free snack.

But that free meal comes at a price: it’s all too easy for a seabird to get caught up in the fishing gear that trails behind a fishing boat, and when they do, it’s usually fatal. In 2004-2005, an estimated 15,500 birds were killed that way off of South Africa. Pelagic albatrosses and petrels are particularly hard-hit, with two species – Black-browed albatross (Thalassarche melanophris) and Shy-type albatross (Thalassarche cauta) – anchoring the top of the list. As a group, seabirds are among the most imperiled, with 28 percent of species classified by the IUCN as vulnerable or endangered. The affected albatrosses are no different; the Black-browned is endangered and the Shy-type fares slightly better as “near threatened.”

The commercial fisheries aren’t going to stop their business for the seabirds. The deep-water hake trawl fishery is the most valuable in South Africa, targeting both Cape hakes (Merluccius paradoxus) and Shallow-water hakes (Merluccius capensis). Following the imposition of “total allowable catches” on the fishery in the late 1970s, trawlers bring in some 120 to 160 thousand tons each year.

A group of researchers led by B.A. Maree of BirdLife South Africa reported this week in the journal Animal Conservation that they’ve uncovered a simple solution. It managed to reduce avian mortality from commercial trawlers by 90 percent, and it costs just $200.

From 2006 to 2010, commercial trawlers were required to deploy “bird scare lines” or BSLs. They’re really quite simple: a thirty meter rope extends from the back of the boat, attached to a floating weight that sits at the water’s surface. At two-meter intervals, small colored streamers hang down from the rope. As the ship moves forward, the line is pulled taut, usually parallel with the trawl cables themselves. The streamers act as a visual deterrent, keeping the birds away.

Can it really be that simple? It appears so. But a closer look reveals that what’s better for the larger seabirds like albatrosses isn’t better for everyone. It turns out that the BSLs simply shifted the risk from larger birds to smaller species. “This may reflect the fact that BSLs are effective at keeping albatrosses out of the danger area, which removes foraging competition in the direct vicinity of the trawl cables,” allowing the smaller birds access to the discarded fish parts, write the researchers.

For example, there were no Pintado petrels (Daption capense) recorded as being killed in 2004-2005, but when BSLs were used they accounted for half (21 of 41) of all avian mortalities in this study. Seventeen of the forty-one birds killed in this study come from species that are of conservation concern (though the Pintado petrels themselves are of “least concern”).

“While it is gratifying to note that the albatrosses (of highest conservation concern) are at substantially reduced risk,” Maree says, “concerns remain for other vulnerable species observed to be killed, including White-chinned petrels and Cape gannets, both listed as vulnerable.” When scaled up, the researchers estimate that some 400 individuals from threatened species could still be killed each year. That’s a significant improvement over the 15,000 annual avian mortality rate without BSLs, but not perfect.

The best thing may simply be to avoid discarding fish guts onto the water’s surface in the first place, thereby avoiding the attention of the scavenging seabirds. One option may be to discharge fish waste directly into the water beneath the surface, through the hull. “Although retrofitting vessels for discard management may be prohibitively expensive,” the researchers acknowledge, “we suggest that all new vessels entering trawl fisheries comply with strict standards that prevent seabirds from accessing discards near the cable– water interface, including prohibitions on discarding while fishing gear is in the water.” – Jason G. Goldman | 9 May 2014